Women spend one third of their lives after menopause. Estrogen replacement therapy and hormone replacement therapy (estrogen and progesterone) have been shown to decrease morbidity and mortality, the symptoms of menopause, the severity of osteoporosis, the risk of coronary artery disease and possibly affect the onset of Alzheimer's disease in postmenopausal women (1)(2). Estrogen replacement therapy has been shown to increase the incidence of endometrial hyperplasia and endometrial carcinoma w/a less well defined effect on the incidence of breast cancer. Various forms of estrogen are some of the most frequently prescribed drugs, however, there is relatively little information regarding factors that affect the metabolism of estrogen. The purpose of this pilot study is to investigate various drugs that may alter the metabolism of estrogen. To determine the effect of an inhibitor (erythromycin) and an inducer (rifampin) of P4503A4 on the pharmacokinetics of estradiol.